1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing and, more specifically, to an improved method and apparatus for chemical-mechanical polishing.
2. Description of Relevant Art
Nonplanar surfaces, when present in integrated circuits having complex, high density multilevel interconnections, may cause the optical resolution of photolithographic processing steps to be poor, which could inhibit the printing of high density lines. Another problem that nonplanar surface topography may cause relates to step coverage of metal layers. If steps are too high or uneven, open circuits could be created. It is thus important, when making such complex integrated circuits, to planarize the surface of many of the layers that make up the device.
Various techniques have been developed to planarize certain layers formed during the process of making integrated circuits. In one approach, known as chemical-mechanical polishing, protruding steps, such as those that may be formed along the upper surface of interlayer dielectrics ("ILDs"), are removed by polishing. Chemical-mechanical polishing may also be used to "etch back" conformally deposited metal layers to form planar plugs or vias.
In a typical chemical-mechanical polishing method, a silicon substrate or wafer is placed face down on a rotating table covered with a flat polishing pad, which has been coated with an active slurry. A carrier, which may be made of a thick nonflexible metal plate that is attached to a rotatable shaft, is used to apply a downward force against the backside of the substrate. A retaining ring may be used to center the substrate onto the carrier to prevent it from slipping laterally. A resilient carrier pad, positioned between the metal plate and the substrate, typically is used to press against the backside of the substrate. By applying the downward force, while rotating the slurry covered pad for a selected amount of time, a desired amount of material may be removed from the upper surface of the thin film to planarize it.
A variation of the above described method, where a uniform pressure is applied to the backside of a wafer to improve polishing uniformity, is described in copending U.S. patent application Ser. No. 08/103,918, filed Aug. 6, 1993, assigned to this application's assignee. Although such a method generally ensures that a uniform pressure will be provided across the surface of a wafer, regardless of polishing pad or table irregularities, at times it may be desirable to vary the pressure applied to the wafer at different locations. For example, if the slurry applied to the polishing pad is thicker near the edges of the wafer than at the wafer's center, one may wish to apply a higher pressure to the center of the wafer than at the edge. One may similarly wish to vary the pressure applied to different portions of the wafer to account for uneven polishing pad wear, or differences in the rate of removal of material from the wafer at different regions of the wafer.
Accordingly, there is a need for an improved chemical-mechanical polishing method and apparatus that enables the user to vary the pressure applied to different regions of the wafer in a controlled manner, when desirable to enhance polishing uniformity.